Terephthalic acid (PTA) is one of the monomers used for the synthesis of the polyester, polyethylene terephthalate (PET), that is used for the large-scale manufacture of synthetic fibers and plastic bottles. PTA is largely produced from the liquid-phase oxidation of petroleum-derived p-xylene (PX). However, there are now ongoing worldwide efforts exploring alternative routes for producing PTA from renewable, biomass resources.
An important biomass-derived furan, 5-hydroxymethylfurfural (HMF), is a valuable platform chemical being considered for the production of many fuels and chemicals, and can be obtained from glucose and other biomass feedstocks. HMF is now being manufactured at the commercial scale in Europe at 20 tons annually using a hydrothermal process. Thus, finding efficient routes to PTA from HMF is of great interest.
One approach to the synthesis of biomass-derived PTA has been to use a Diels-Alder reaction between a biomass-derived furanic diene and a dienophile as a way to build the six carbon ring necessary for the synthesis of PTA. The Diels-Alder adduct is an oxabicyclic intermediate that must be further dehydrated to achieve the aromatic product. Diels-Alder routes to PTA from HMF have been investigated, and typically, they require the hydrogenation of HMF to 2,5-dimethylfuran (DMF). It is known that DMF and ethylene can react to form p-xylene in a one-pot Diels-Alder-dehydration reaction (see FIG. 1, top reaction scheme). This reaction has received much attention recently, and it is catalyzed by homogeneous Lewis acids and a wide variety of heterogeneous, Brønsted acid-containing solids. Although nearly 100% p-xylene yields from the Diels-Alder-dehydration reaction can be achieved, the reduction step to convert HMF to DMF requires expensive metal catalysts and a hydrogen source, making this route challenging to commercially implement. Therefore, the discovery of routes to PTA from HMF that avoid any hydrogenation steps could be useful.
Another oxidation route to PTA that has previously been considered is the Diels-Alder-dehydration reaction of ethylene with the fully oxidized HMF, 2,5-furandicarboxylic acid (FDCA). This reaction is extremely slow and only negligible yields have been reported. The unreactivity of FDCA in the Diels-Alder-dehydration reaction is presumably due to the strong electron-withdrawing effects of the two carboxyl (—CO2H) groups that result in a very electron-poor and deactivated diene.
There is significant interest and a high need in the synthesis of renewable, biomass-derived terephthalic acid.